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A086714
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a(1) = 4, a(n) = a(n-1)*(a(n-1) - 1)/2.
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5
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OFFSET
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1,1
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COMMENTS
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The next two terms, a(10) and a(11), have 111 and 221 digits. - Harvey P. Dale, Jun 10 2011
Interpretation through plane geometry: Start with the a(n)-sided regular polygon, connect all the vertices to create a figure having a(n+1)=A000217(a(n)-1) edges. Repeat to obtain this sequence. - T. D. Noe, May 13 2016
Let y(1) = x1+x2+x3+x4, and define y(n+1) as the plethysm e2[y(n)], where e2 represents the second elementary symmetric function. Then a(n) is y(n) evaluated at x1=x2=x3=x4=1. - Per W. Alexandersson, Jun 06 2020
Each term is the number of coordinate planes in Euclidean space of the dimensionality of the previous term. - Shel Kaphan, Feb 06 2023
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LINKS
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FORMULA
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Limit_{n->oo} a(n)^(1/2^n) = 1.280497808541657066685323460209089278782... (see A251794). - Vaclav Kotesovec, Feb 15 2014, updated Dec 09 2014
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EXAMPLE
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a(2) = a(1)*(a(1)-1)/2 = 4*3/2 = 6.
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MATHEMATICA
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RecurrenceTable[{a[1]==4, a[n]==(a[n-1](a[n-1]-1))/2}, a[n], {n, 10}] (* Harvey P. Dale, Jun 10 2011 *)
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PROG
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(PARI) v=vector(10, i, (i==1)*4); for(i=2, 10, v[i]=v[i-1]*(v[i-1]-1)/2); v
(PARI) a086714(upto)={my(a217(n)=n*(n+1)/2, a=4); for(k=1, upto, print1(a, ", "); a=a217(a-1))};
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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